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Hawaiian Islands Synoptic Discussion and Guidance

858
FXHW60 PHFO 301631 AAA
AFDHFO

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HONOLULU HI
631 AM HST WED JUL 30 2014

.UPDATE...
INCREASED CHANCES FOR CLOUDS AND RAIN TODAY OVER OAHU AND MOLOKAI
DUE TO THE INCOMING FETCH OF CLOUDS CARRIED IN WITH THE TRADE
WINDS. ORIGINAL FORECAST HAD RAIN TARGETED OVER KAUAI...BUT WILL
LIKELY SPREAD A BIT FURTHER SOUTH GIVEN CURRENT SATELLITE LOOP. HIGHER
RAIN CHANCES WILL MAINLY AFFECT NORTH SHORES...WINDWARD AREAS AND
THE MOUNTAINS. LEEWARD SIDES COULD SEE AN ISOLATED SHOWER OR
TWO...BUT SHOULD REMAIN MOSTLY DRY.


&&

.SYNOPSIS...
HIGH PRESSURE FAR NORTH OF THE STATE WILL KEEP TRADE WINDS BLOWING
THROUGH THE WEEKEND. DRY CONDITIONS INTO THURSDAY WILL YIELD TO A
VERY MOIST AIR MASS BRINGING INCREASING SHOWERS FROM THE SOUTHEAST
BY FRIDAY EVENING. CONDITIONS WILL DRY OUT AGAIN SLOWLY OVER THE
WEEKEND AND INTO NEXT WEEK AS THE MOIST AIR MASS MOVES AWAY TOWARD
THE WEST.

&&

.PREV DISCUSSION.../ISSUED 400 AM HST WED JUL 30 2014/

SURFACE OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS STILL REPORTED
GENERALLY MODERATE TRADE WINDS...THOUGH AS USUAL SPEEDS WERE HIGHER
WHERE THE FLOW ACCELERATED AROUND HEADLANDS AND BETWEEN ISLANDS.
CLOUD COVER WAS LIMITED AND PRECIPITATION QUITE SPARSE OVERALL...
BUT TWO EXCEPTIONS WARRANTED MENTION. FIRST...SELECT LOWER WINDWARD
SLOPES OF THE BIG ISLAND MANAGED TO RECEIVE UPWARDS OF HALF AN INCH
OF RAIN AROUND MIDNIGHT...AND MUCH LIGHTER PRECIPITATION SINCE.
SECOND...LIGHT SHOWERS MOVED ASHORE OVER WINDWARD SHORES OF KAUAI
DURING THE PAST FEW HOURS. THE MOISTER AIR DRIFTING OVER KAUAI FROM
THE NORTHWEST SUGGESTED THAT THE SHOWER ACTIVITY THERE WILL REMAIN
MORE ACTIVE THROUGH THE DAY...AND THE FORECAST HAS BEEN ADJUSTED
ACCORDINGLY...THOUGH ACCUMULATION SHOULD REMAIN ON THE LOW SIDE. ON
THE OTHER HAND...THE SHALLOW INVERSION ACROSS ALL THE STATE...AND
THE DRIER AIR MASS FROM OAHU TO THE BIG ISLAND...SHOULD LIMIT
CONTINUED RAINFALL THERE THROUGH THE SHORT RUN.

GUIDANCE AGREED IN PREDICTING GRADUAL CHANGE INTO THE WEEKEND. THE
MAIN TWO TRENDS WILL CONSIST OF A SIGNIFICANTLY MOISTER AIR MASS
ARRIVING FROM THE ACTIVE ITCZ FROM THURSDAY INTO FRIDAY...AND A
MID-LEVEL RIDGE BUILDING IN OVERHEAD FROM THE EAST THROUGH SATURDAY.
THE MOISTER AIR WILL PROVIDE MORE FUEL FOR SHOWERS...BUT THE RIDGE
ALOFT WILL PRODUCE SUBSIDENCE SUPPRESSING CLOUD DEVELOPMENT.
HOWEVER...INCREASED SUPPORT FOR THE SUBTROPICAL RIDGE AT THE SURFACE
ALSO WILL BRING SOMEWHAT FASTER WIND SPEEDS...PROVIDING MORE
OROGRAPHIC FORCING ALONG WINDWARD SLOPES. FINALLY...IF ENOUGH OF THE
MOIST AIR MASS ARRIVES BEFORE THE RIDGE ALOFT REALLY STARTS TO
BUILD...THEN A BROAD BUT SHALLOW MID-LEVEL TROUGH LINGERING TO THE
NORTH NORTHEAST MAY HAVE A BRIEF OPPORTUNITY TO TRIGGER CONVECTION
OVER THE STATE.

THIS WINDOW OF OPPORTUNITY APPEARED TO CENTER AROUND FRIDAY NIGHT.
ALTHOUGH LOCAL TOOLS RUN ON MODEL OUTPUT DID NOT PREDICT MUCH CHANCE
OF THUNDERSTORMS...EVENTS DRIVEN PRIMARILY BY LOW-LEVEL MOISTURE
WITH LITTLE SUPPORT FROM ALOFT DID NOT EXACTLY PLAY TO THE STRENGTHS
OF THOSE TOOLS. ALSO...TEMPERATURES ALOFT APPEARED TOO WARM TO FAVOR
DEEP CONVECTION. FOR NOW...THE BEST COMPROMISE APPEARED TO CONSIST
OF SHOWERS INCREASING GRADUALLY FROM THE SOUTHEAST ONTO THE BIG
ISLAND FROM THURSDAY INTO FRIDAY...AND THEN LOCALLY HEAVY RAIN
CREEPING NORTHWEST FROM THE BIG ISLAND TOWARD THE SMALLER ISLANDS
FROM FRIDAY EVENING INTO SATURDAY. THUNDER MAY BE AN OUTSIDE
POSSIBILITY FROM THURSDAY INTO SATURDAY...ESPECIALLY TO THE EAST
THROUGH SOUTH OF THE BIG ISLAND...BUT AT THIS POINT IT HAS BEEN
INTRODUCED INTO THE FORECAST ONLY OFFSHORE TO THE SOUTH OF THE BIG
ISLAND ON THURSDAY AFTERNOON.

BY LATE IN THE WEEKEND...SOLUTIONS STARTED TO TREND TOWARD
CLIMATOLOGY AGAIN. WIND SPEEDS SHOULD DIMINISH SLIGHTLY...WHILE THE
MOIST AIR MASS WILL DEPART TOWARD THE WEST...AS DRIER AIR ADVECTS IN
FROM THE NORTHEAST.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL ACROSS THE MAIN HAWAIIAN ISLANDS
TODAY AND TONIGHT. PASSING SHOWERS AND ISOLATED MVFR CIGS WILL
BE POSSIBLE ON WINDWARD SLOPES. AIRMET TANGO REMAINS IN EFFECT FOR
LOW LEVEL TURBULENCE OVER AND IMMEDIATELY DOWNWIND OF THE HIGHER
TERRAIN...DUE TO MODERATE TO LOCALLY BREEZY TRADE WINDS.

&&

.MARINE...
A SMALL CRAFT ADVISORY REMAINED IN EFFECT FOR TYPICALLY WINDY WATERS
AROUND MAUI COUNTY AND THE BIG ISLAND. THESE CONDITIONS ARE EXPECTED
TO CONTINUE INTO THE WEEKEND...SO THE ADVISORY LIKELY WILL BE
EXTENDED.

A SERIES OF SMALL SOUTHERLY SWELLS SHOULD ARRIVE DURING THE NEXT FEW
DAYS...BUT A LARGER SOUTH SWELL MAY REACH THE STATE OVER THE
WEEKEND. TRADE WINDS WILL CONTINUE TO PRODUCE SHORT-PERIOD CHOPPY
SURF ALONG EAST-FACING SHORES.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 PM HST FRIDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.


&&

$$

UPDATE...FOSTER
DISCUSSION/MARINE...RYSHKO
AVIATION...JACOBSON


Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office